I have the same problem. During day watches monitor HR well. If I start some sport, it does not matter what type of sport, watches stops the monitoring. I am trying to use Samsung Healt, Endomondo, and it does not helped.

Ah, that's a shame it didn't work for you. It has virtually solved it for me. I know it looks a bit nuts, but I only wear it like that for exercise. And I kinda like it now, the watch is nearer to you for a quick glance at your stats.

Well after weeks of frustration I reset to factory and shaved the area of wrist under the watch and it has not failed yet. I always leave it on "always" for monitoring. I just wish Samsung would come up with an update as the watch is capabable of doing what is required.